Comment 1 for bug 1701399

It isn't non-existent windows drives. It is removable drives with
missing media, and is caused by this python bug:

Fundamentally, this is a windows issue, there is no reliable way to
check for the existence of a file without popping up that error dialog.
I have changed calibre to call SetErrorMode at startup to try to
suppress that popup. However, lots of other software also calls
SetErrorMode all the time, so there is no guarantee that doing so will
actually work on all systems. However, it should reduce the probability
of it happening.

 status fixreleased